1. Field of the Invention
The present invention relates to an image forming apparatus having a cartridge incorporated therein.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a photosensitive member as a rotatable image bearing member, and outputs images by transferring toner images formed on the photosensitive member to sheets. In such an image forming apparatus, because of use over time, the photosensitive member and image forming units configured to form toner images on the photosensitive member become deteriorated and worn. Thus, these components require regular replacement (maintenance).
To facilitate the maintenance, a product (hereinafter, referred to as a cartridge) that is a package of disposable components such as a photosensitive member is commercially available. In many cases, the cartridge is replaceably mounted to an image forming apparatus. The photosensitive member is rotatable by driving force supplied from the main body of the apparatus to form images.
In the replaceable cartridge, the photosensitive member is rotated by driving force supplied from the apparatus main body. The driving force is supplied from the apparatus main body to the cartridge through a coupling, which is a receiving member for the driving force. The coupling requires a clearance (play) to allow attachment/detachment of the cartridge to/from the apparatus main body. In such a structure, if the rotational axis of the coupling of the cartridge is positioned offset from the central axis of a coupling of the apparatus main body, a variation in rotation speed of the photosensitive member may be caused. The variation in rotation speed of the photosensitive member then results in image defects such as defective transfer and image distortion.
Japanese Patent Application Laid-Open No. 2006-145774 discusses a structure in which the rotation shaft of a coupling of a cartridge and the rotation shaft of a coupling of an apparatus are precisely aligned to suppress a variation in rotation speed of a photosensitive member.
In recent years, it has been found that a reduction in speed difference between a photosensitive member, as an image bearing member, and a transfer-receiving member (e.g., a sheet or an intermediate transfer member) improves transferability of images. More specifically, a reduction in difference between the speed of a photosensitive member and the speed of a transfer-receiving member in an image forming apparatus can suppress image defects (hereinafter, referred to as white spots), which involve no transfer of part of a toner image born on the photosensitive member onto the transfer-receiving member.
In the structure discussed in Japanese Patent Application Laid-Open No. 2006-145774, the cartridge is supported by a drive shaft of the apparatus and a coupling is provided on the shaft. A reduction in speed difference between the photosensitive member and the transfer-receiving member may cause a problem as follows.
The problem is described with reference to FIGS. 10A and 10B. FIGS. 10A and 10B each illustrate a rotation speed of a photosensitive member and a moving speed of an intermediate transfer member. The photosensitive member and the intermediate transfer member each have a given component tolerance, which periodically varies the rotation speed of the corresponding member. As illustrated in FIG. 10A, in the case with a large difference in speed between the photosensitive member and the intermediate transfer member as a transfer-receiving member, notwithstanding the periodic variation in speed, the photosensitive member constantly rotates faster than the intermediate transfer member. As illustrated in FIG. 10B, however, in the case with a small difference in speed between the photosensitive member and the intermediate transfer member, the intermediate transfer member rotates momentarily faster than the photosensitive member (the period “reverse” in FIG. 10B). In the reverse state of speed, the photosensitive member receives a force from the intermediate transfer member in a direction to cause separation of the coupling. In the structure where the cartridge is supported by a drive shaft of the apparatus and a coupling is provided on the shaft, the force exerted in that direction results in momentary separation of the coupling although the separation is well recovered soon. Accordingly, a variation in speed of the photosensitive member and the transfer-receiving member generates a force applied to the photosensitive member and causes the coupling to be momentarily separated, resulting in a state where driving force is not transmitted to the photosensitive member. This precludes the achievement of satisfactory transferability of images.